This invention relates to processes and systems for cementing casing in a wellbore. The invention more particularly relates to a reverse circulation process wherein cement is pumped down the annulus between the casing and the wellbore and held in place while the cement hardens.
Present cementing processes typically pump a cement slurry down the inside of the casing, out the casing shoe, and up the annulus. Rubber plugs are displaced down the casing behind the slurry to prevent the slurry from depositing inside the casing. Because the cement must travel all the way to the bottom of the casing, to the shoe, and then back up the casing-by-bore annulus, expensive cement retarders are mixed with the cement slurry to ensure the cement does not set prematurely. The long trip also makes for long pump times.
Cement slurries are relatively dense and heavy fluids. To lift the slurry above the casing shoe in the annulus, high-pressure pumping equipment must be used to pressurize the casing. The high pressure drives the cement slurry and wiper plug down the casing and out through the casing shoe into the annulus. High pressure within the casing may cause fractures and other damage to the casing. Further, the high pressure generated in the annulus in the bottom of the bore hole can be sufficient to drive the cement slurry into the formation resulting in formation breakdown.
Alternatively, a reverse circulation method has been used where the cement slurry is pumped down the casing-by-bore annulus. The slurry is displaced down the annulus until the leading edge of the slurry volume is just inside the casing shoe. The leading edge of the slurry must be monitored to determine when it arrives at the casing shoe. Logging tools and tagged fluids (by density and/or radioactive sources) have been used monitor the position of the leading edge of the cement slurry. If significant volumes of the cement slurry enters the casing shoe, clean-out operations must be conducted to insure that cement inside the casing has not covered targeted production zones. Position information provided by tagged fluids is typically available to the operator only after a considerable delay. Thus, even with tagged fluids, the operator is unable to stop the flow of the cement slurry into the casing through the casing shoe until a significant volume of cement has entered the casing. Imprecise monitoring of the position of the leading edge of the cement slurry can result in a column of cement in the casing 100 feet to 500 feet long. This unwanted cement must then be drilled out of the casing at a significant cost.